The human body and cancer
Cancer cells grow aggressively and invade other neighbouring tissues. A few cancer cells rapidly become many cells and some cells break away from the primary tumour and settle in distant places, where they form new secondary tumours (or metastases, pronounced 'mett-ass-tah-seez'). As a primary or secondary cancer gets larger, it releases chemical signals that cause new blood capillaries to grow into it.
Why do you think a cancer needs new blood vessels as it grows larger?
The cancer cells in the middle of the tumour need a supply of oxygen and nutrients to fuel their growth. The new blood vessels bring it to them and remove waste products such as carbon dioxide.